1. Field of the Invention
The present invention generally relates to the degradation that occurs when compound semiconductor Field Effect Transistors (FETs) having gates that include Pt or Pd barrier layers are exposed to hydrogen, and more specifically to a method for pretreating the FETs to substantially prevent such degradation.
2. Description of the Related Art
Integrated circuits (ICs) are often hermetically packaged to improve their reliability, extend their lifetimes and improve their performance. The hermetic package prevents moisture from coming in contact with the devices and degrading their performance. Hermetic packaging is required in many space subsystems and military applications.
The ICs produce heat when operated, which over a period of time causes hydrogen that is trapped in the hermetic packaging to be released within the package. Most devices are unaffected by the hydrogen release. However FETs that are formed on a compound semiconductor substrate such as InP or GaAs, and have gates that include Pt or Pd barrier layers (typically TiPtAu or TiPaAu gates), degrade when exposed to a hydrogen atmosphere. The FET's degradation is sudden and dramatic, causing the device to fail far short of its expected lifetime. Hermetically packaged hydrogen-sensitive FETs have an expected lifetime of only approximately 3 years as a result of the hydrogen effect. By contrast, unpackaged FETs of the same type have lifetimes of approximately 15 years.
FETs affected by hydrogen exposure are used extensively in radio frequency (RF) subsystems for low noise amplifiers, power amplifiers and microwave monolithic integrated circuits (MMICs). One example of such an FET is a GaInAs high electron mobility transistor (HEMT), which is described in Mishra et al., "Microwave Performance of AlInAs-GaInAs HEMT's with 0.2- and 0.1-.mu.m Gate Length", IEEE Electron Device Letters, Vol. 9, No. 12, Dec. 1988, pp. 647-649.
One explanation for the cause of the degradation is offered in Camp et al., "Hydrogen Effects on Reliability of GaAs MMICs" Semiconductor Device Reliability, 1990, pp. 471-477. The authors believe that the Pt or Pd barrier metal acts as a catalyst to ionize the molecular hydrogen, which then recombines with the ionized silicon donors in the channel. In this manner the charge carriers are depleted from the channel, causing the device's drain current to drop. Camp et al show that the degradation effects are not permanent and that a significant portion of the current loss can be temporarily restored by heating the device in a 100% nitrogen atmosphere. However, the current restoration is not permanent and further degradation occurs after the device has been hermetically sealed.
In applications such as satellite communications and military products, device performance and lifetime is very important and replacement is often impractical. In these systems the FETs are typically fabricated with TiAl or WAu gate metalizations, which are not affected by the exposure to hydrogen. However, these FETs exhibit a relatively high electron migration that limits their lifetime to approximately 10 years, thus limiting the lifetime of the satellite or other products into which they are incorporated and increasing its effective cost. Additionally, these devices are not efficient at higher power levels, and hence will limit the performance of higher powered satellites. In mass produced commercial applications, failure is not critical and replacement is possible, although it can be time consuming and expensive.
GaInAs HEMTs and in general the hydrogen-sensitive compound semiconductor FETs are relatively easy to fabricate, have long lifetimes (typically 15 years) due to relatively low electron migration and can handle relatively high power levels. Exposing the FETs to hydrogen causes degradation that prevents their use in many applications and increases their cost in others. While it is possible to avoid the problem by removing the hydrogen from the hermetic package, current techniques for doing this are very difficult, time consuming and expensive. Therefore it would be highly desirable to find a simple and cost efficient method for making hydrogen-sensitive FETs impervious to hydrogen exposure without degrading their electrical properties.